Method and apparatus for limiting maximum output power of a power converter

ABSTRACT

An example power converter includes an energy transfer element, a switch, a controller, and a current offset circuit. The controller is coupled to switch the switch between an ON state and an OFF state to regulate the output of the power converter. The controller is also adapted to terminate the ON state of the switch in response to a switch current flowing through the switch reaching a switch current threshold. An auxiliary winding of the energy transfer element is adapted to generate an auxiliary winding voltage that is representative of an input voltage of the power converter only during the ON state of the switch. The current offset circuit is coupled to the auxiliary winding to generate an offset current to flow through the switch in response to the auxiliary winding voltage, where an input current of the power converter is adjusted in response to the offset current.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/422,823, filed Apr. 13, 2009, which is now pending. U.S. patentapplication Ser. No. 12/422,823 is hereby incorporated by reference.

BACKGROUND INFORMATION

The present invention relates generally to power converters, and morespecifically, the invention relates to control circuits that limit amaximum output power of a power converter.

BACKGROUND

Many electrical devices such as cell phones, personal digital assistants(PDAs), laptops, etc. are powered by a source of dc power. Because poweris generally delivered through a wall outlet as high-voltage ac power, adevice, typically referred to as a power converter or power supply, isrequired to transform the high ac voltage to low dc voltage to supplyelectrical power to many electrical devices. In operation, a powerconverter may use a controller to regulate output power delivered to anelectrical device that may be generally referred to as a load. In oneinstance a controller may control a transfer of energy pulses created byswitching a power switch on and off in response to feedback informationof an output voltage to keep the output voltage at the output of thepower converter regulated.

In certain applications, a power converter may be designed to operateunder a wide range of input voltages. Typically, components of powerconverters are capable of delivering significantly more power when thepower converter is coupled to a high ac input voltage than a low acinput voltage. For instance, there may be one application in which thepower converter is connected to a load that requires up to 15 W. Howeverthe load also may be specified to receive no more than 20 W at any time.In this application, the power converter may be designed to deliver, atmost, the maximum power required by the load (15 W) when connected to arelatively low input voltage, for example 85 VAC. However, when thepower converter is connected to a higher input voltage, for example, 265VAC, the power delivered to the output of the power converter mayincrease to greater than 20 W. This could lead to excess current flowingthrough the load (electrical device coupled to the power converter)during a fault condition, which could create damage to the electricalcircuitry in the load. More specifically, a fault condition may bedefined as when the power converter looses ability to regulate. Forexample, a fault condition may include a situation when the output ofthe power converter is overloaded.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments and examples of the presentinvention are described with reference to the following figures, whereinlike reference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1A is a functional block diagram illustrating an example powerconverter, in accordance with the teachings of the present invention.

FIG. 1B illustrates a continuous conduction mode waveform of atransformer current, in accordance with the teachings of the presentinvention.

FIG. 1C illustrates discontinuous and continuous conduction modewaveforms of a transformer current, in accordance with the teachings ofthe present invention.

FIG. 2 is a functional block diagram illustrating an example powerconverter including an integrated circuit, in accordance with theteachings of the present invention.

FIG. 3 is a functional block diagram illustrating an example powerconverter, in accordance with the teachings of the present invention.

FIG. 4A is a graph illustrating an effect of a current limit offset onswitch current, in accordance with the teachings of the presentinvention.

FIG. 4B is a graph illustrating a relationship between input voltage andcurrent limit offset, in accordance with the teachings of the presentinvention.

FIG. 5 is a functional block diagram illustrating an example powerconverter including an example controller, in accordance with theteachings of the present invention.

FIG. 6 is a functional block diagram illustrating an example powerconverter, in accordance with the teachings of the present invention.

DETAILED DESCRIPTION

A method and apparatus to limit the maximum output power of a powerconverter is disclosed. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one havingordinary skill in the art that the specific detail need not be employedto practice the present invention. In other instances, well-knownmaterials or methods have not been described in detail in order to avoidobscuring the present invention.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. In addition, it is appreciated that the figures providedherewith are for explanation purposes to persons ordinarily skilled inthe art and that the drawings are not necessarily drawn to scale.

In short, embodiments of the present invention include a power converterthat limits the maximum power delivered to its output under certainconditions to prevent damage to an attached electrical device.Therefore, embodiments of the present invention provide adequate powerto the load under one set of input voltage conditions and may alsoprovide limiting the maximum power delivered to the load under adifferent set of input voltage conditions. That is, a feature of a powerconverter in accordance with the teachings of the present invention maybe used to provide a minimum power delivery capability while limitingthe maximum power delivery capability when the power converter isdesigned to operate over a wide range of input voltage conditions.

Furthermore, a power converter in accordance with the teachings of thepresent invention may limit the maximum output power without directlymeasuring the input voltage of the power converter. That is, embodimentsdisclosed herein include a controller that requires no additionalterminals to measure the input voltage nor additional terminals foradjusting a current limit threshold of the controller. Consequently, apower converter designed to operate with multiple ranges of inputvoltages with a controller that includes a reduced number of terminalstranslates into reduced costs.

FIG. 1A is a functional block diagram illustrating an example of a powerconverter 100, in accordance with the teachings of the presentinvention. The illustrated example of power converter 100 includes acontroller 102, a power switch 104, a current offset circuit 106, apower circuit 108, an energy transfer element 112, a feedback circuit114, an output diode 116, an output capacitor 118, and a current sensor124. In one example power switch 104 is a MOSFET and current sensor 124is the on resistance of the MOSFET that generates a current sense signalU_(SENSE) that is a voltage generated in response to a current I_(SW)flowing in power switch 104. In operation, power converter 100 providesoutput power to a load 119 from an unregulated dc input voltage V_(IN).In one example, dc input voltage V_(IN) may be the result of rectifyingan ac voltage (not shown) delivered through a wall outlet. As shown,power converter 100 is configured as a flyback converter. In oneexample, power converter 100 may also be configured as, but not limitedto, a forward converter, a buck converter, a buck-boost converter, aSEPIC converter, a Cuk converter, or a flyforward converter. In theexample of FIG. 1A, energy transfer element 112 is a coupled inductor,sometimes referred to as a transformer, with input winding 120 andoutput winding 122. An “input winding” may also be referred to as a“primary winding” and an “output winding” may also be referred to as a“secondary winding.” In one example, energy transfer element 112 may begalvanically isolated. More specifically, galvanic isolation prevents dccurrent from flowing between the input side and the output side of thepower converter, and may be required to meet safety regulations. Asshown, an input return 105 is electrically coupled to circuitry on the‘input side’ of power converter 100. Similarly, an output return 107 iselectrically coupled to circuitry on the ‘output’ side of powerconverter 100.

In one example, primary winding 120 is coupled to power switch 104 suchthat, in operation, energy transfer element 112 receives energy with aninput current I_(IN) when power switch 104 is in an ‘ON’ state andenergy transfer element 108 delivers energy to the output of powerconverter 100 when power switch 104 is in an ‘OFF’ state. Duringoperation, power switch 104 may be switched between the ON state,thereby allowing switch 104 to conduct current, and the OFF state,thereby preventing switch 104 to conduct current. As shown, controller102 outputs a switching signal U_(SW) to switch power switch 104 betweenan ON state and an OFF state. In one example, power switch 104 is ametal oxide semiconductor field effect transistor (MOSFET). In anotherexample, controller 102 may be implemented as a monolithic integratedcircuit or may be implemented with discrete electrical components or acombination of discrete and integrated components.

During operation of power converter 100, the switching of power switch104 produces pulsating current in output diode 116 which is filtered byoutput capacitor 118 to produce a substantially constant output voltageV_(OUT). In one example, the switching of power switch 104 may provide asubstantially constant output current I_(OUT) to a load 119 coupled tothe output of power converter 100. As shown, feedback circuit 114 iscoupled to provide a feedback signal U_(FB) to controller 102. In oneexample, an output quantity to be regulated by controller 102 andfeedback circuit 114 may be output voltage V_(OUT). According to anembodiment of the present invention, feedback circuit 114 may implementan indirect sensing of the output quantity to regulate an outputquantity of power converter 100. In one example, a bias winding that iselectrically coupled to the ‘input side’, may be magnetically coupled tothe output side of power converter 100 to sense an output quantity atthe output of power converter 100. During operation, feedback signalU_(FB) may be representative of an error voltage which may be defined asa difference between an output voltage V_(OUT) and a desired outputvoltage of power converter 100. In one example, controller 102 includesan oscillator (not shown) that defines substantially constant switchingperiods T_(S) during which power switch 104 may be conducting or notconducting. In one example, for power switch 104 to regulate the outputvoltage V_(OUT), controller 102 determines whether or not the powerswitch 104 will be allowed to conduct during a switching period T_(S) inresponse to the feedback signal U_(FB). A switching period T_(S) whereinthe switch 104 is allowed to conduct is an ‘enabled’ period. A switchingperiod T_(S) wherein the switch 104 is not allowed to conduct is a‘disabled’ period. In other words, controller 104 decides to eitherenable or disable power switch 106 during each switching period T_(S) tocontrol the transfer of energy to the output of power converter 100. Inthis manner, controller 102 may regulate the output voltage V_(OUT) ofpower converter 100 in response to feedback signal U_(FB).

As shown in the depicted example, a current sensor 124 is coupled tosense a switch current I_(SW) flowing through power switch 104. Morespecifically, current sensor 124 may be implemented using a currenttransformer, or a discrete resistor, or a main conduction channel of atransistor when the transistor is conducting, or a senseFET elementforming part of a transistor. During operation, current sensor 124generates a current sense signal U_(SENSE) that is representative ofswitch current I_(SW). In one example, current sense signal U_(SENSE) isused by controller 102 to limit switch current I_(SW) in power switch104 during each switching period T_(S). In other words, current sense124 is used to indicate when switch current I_(SW) exceeds a currentlimit threshold. In this manner, switch 104 is switched from an ON stateto an OFF state when the current limit threshold of power switch 104 isreached. That is, the ON state of switch 104 may be terminated inresponse to switch current I_(SW) reaching the current limit threshold.

Controller 102 may be further characterized, according to the teachingsof the present invention, to switch power switch 104 at a constantfrequency or variable frequency. A constant frequency occurs when eachswitching period T_(S) of switch 104 is controlled to be substantiallyconstant. A variable frequency switching period occurs when theswitching period T_(S) of switch 104 is responsive to a feedback signalU_(FB). In one example controller 102 can use a combination of constantfrequency or variable frequency modes of operation to regulate an outputof power converter 100 depending on the specific operating conditions.

Referring now to FIG. 1B, waveforms associated with a continuousconduction mode of operation for controller 102 are illustrated inaccordance with the teachings of the present invention. As shown, FIG.1B illustrates a switch waveform 180 b and an energy waveform 185 b.More specifically switch waveform 180 b is representative of a switchcurrent I_(SW) through power switch 104 and energy waveform 185 b isrepresentative of the energy stored in energy transfer element 112. Asshown in waveform 185 b, when operating in continuous conduction modeenergy in energy transfer element 112 does not reach zero within aswitching period T_(S) because power switch 104 turns on before all theenergy is transferred to the output of power converter 104. In oneexample, the amount of energy delivered per switching period, whencontroller 102 is implementing continuous conduction mode, may bequantitatively defined using the following relationship:E _(TRANSFERRED) =E ₁ −E ₂  (EQ. 1)

where E₁ is representative of the amount of energy stored at the end ofan on time T_(ON) during a switching period T_(S), and E₂ isrepresentative of the amount of energy stored in energy transfer element112 at the end of an off time T_(OFF) during a switching period T_(S),as shown in waveform 185 b. E₁ may be quantitatively defined using thefollowing relationship:

$\begin{matrix}{E_{1} = {\frac{1}{2}{LI}_{LIMIT}^{2}}} & ( {{EQ}.\mspace{14mu} 2} )\end{matrix}$

where L is the inductance of primary winding 120, and I_(LIMIT) is themaximum switch current I_(SW) when power switch 104 is able to conduct.

E₂ may be quantitatively defined using the following relationship:

$\begin{matrix}{E_{2} = {\frac{1}{2}{LI}_{INITIAL}^{2}}} & ( {{EQ}.\mspace{14mu} 3} )\end{matrix}$

where L is the inductance of primary winding 120, and I_(INITIAL) is theinitial amount of switch current I_(SW) at the beginning of a subsequentenabled cycle.

In one example, controller 102 operates in continuous conduction modewhile maintaining a substantially constant switching period T_(S). Asthe on time T_(ON) of a constant switching period T_(S) decreases, theoff time T_(OFF) of that constant switching period T_(S) will increase.More specifically, the on time T_(ON) may be caused to decrease when theinput voltage V_(IN) is increased. This may occur because switch currentI_(SW) through power switch 104 increases at a faster rate at higherinput voltages V_(IN), therefore allowing switch current I_(SW) to reachcurrent limit I_(LIMIT) in a shorter time. Since energy from energytransfer element 112 is delivered to the output of power converter 100during the off time T_(OFF) of a switching period, the longer the offtime T_(OFF), the more energy that is delivered to the output of thepower converter during a switching period T_(S).

In one example, controller 102 operates in continuous conduction modewhile maintaining a substantially constant switching period T_(S). Asthe on time T_(ON) of a constant switching period T_(S) decreases, theoff time T_(OFF) of that constant switching period T_(S) will increase.More specifically, the on time T_(ON) may be caused to decrease when theinput voltage V_(IN) is increased. This may occur because switch currentI_(SW) through power switch 104 increases at a faster rate at higherinput voltages V_(IN), therefore allowing switch current I_(SW) to reachcurrent limit I_(LIMIT) in a shorter time. Since energy from energytransfer element 112 is delivered to the output of power converter 100during the off time T_(OFF) of a switching period, the longer the offtime T_(OFF), the more energy that is delivered to the output of thepower converter during a switching period T_(S).

Referring now to FIG. 1C, a discontinuous conduction mode of operationfor controller 102 is illustrated in accordance with the teachings ofthe present invention. As shown, FIG. 1C illustrates a switch waveform180 c and an energy waveform 185 c. More specifically switch waveform180 c is representative of a switch current I_(SW) through power switch104 and energy waveform 185 c is representative of the energy stored inenergy transfer element 112 when controller 102 is operating indiscontinuous conduction mode. As shown in waveform 185 c, whenoperating in discontinuous conduction mode, all energy in energytransfer element 112 is transferred within a switching period T_(S). Inone example, the amount of energy delivered per switching period T_(S)when controller 102 is operating in discontinuous conduction mode, maybe quantitatively defined using the following relationship:E _(TRANSFERRED) =E ₁−0  (EQ. 4)

where E₁ is representative of the amount of energy stored at the end ofan on time T_(ON) during a switching period T_(S), as shown in waveform185 b. E₁ may be quantitatively defined using the followingrelationship:

$\begin{matrix}{E_{1} = {\frac{1}{2}{LI}_{LIMIT}^{2}}} & ( {{EQ}.\mspace{14mu} 5} )\end{matrix}$

where L is the inductance of primary winding 120, and I_(LIMIT) is themaximum switch current I_(SW) when power switch 104 is able to conduct.

In one example, controller 102 operates in discontinuous conduction andalso maintains a constant switching period T_(S). During operation, allof the energy received during an on time T_(ON) of a switching periodT_(S) is transferred to the output side of power converter 100 duringthe off time T_(OFF) of a switching period T_(OFF). In one example,power converter 100 may shift from operating in continuous conductionmode at a certain input voltage to operating in discontinuous conductionmode when power converter 100 is introduced to a substantially higherinput voltage. In another example, power converter 100 may be designedto always operate in discontinuous conduction mode. In yet anotherexample, power converter 100 may be designed to always operate incontinuous conduction mode.

Referring back to FIG. 1A, power circuit 108 is coupled to power bothcurrent offset circuit 106 and controller 102. In another example,current offset circuit 106 may receive a source of power from othercircuitry included in power converter 100 or may be separately poweredwith circuitry specifically designed to power offset current circuit106. Power signal U_(POWER) may be substantially representative of inputvoltage V_(IN) and in one example, may be representative of inputvoltage V_(IN) only during the ON state of power switch 104. That is, inone example, current offset circuit 106 is only able to receive powerfrom power circuit 108 during the ON state of power switch 104.

As further shown in FIG. 1A, current offset circuit 106 is coupled toprovide an offset current I_(OFFSET) to power switch 104. A magnitude ofoffset current I_(OFFSET) may be responsive to a magnitude of powersignal U_(POWER). However, as stated above, power signal U_(POWER) maybe representative of input voltage V_(IN) and thus, in one example,offset current I_(OFFSET) may be responsive to input voltage V_(IN). Inone example, the magnitude of offset current I_(OFFSET) is proportionalto a magnitude of input voltage V_(IN). Thus, as input voltage V_(IN)changes, more or less offset current I_(OFFSET) may be provided to powerswitch 104. For example, offset current I_(OFFSET) may have a firstmagnitude when input voltage V_(IN) is low and a higher second magnitudewhen input voltage V_(IN) is relatively higher.

In one example, switch current I_(SW) is the sum of input current I_(IN)and offset current I_(OFFSET). Thus, as the magnitude of offset currentI_(OFFSET) increases, the peak value of input current I_(IN) decreasessince switch current I_(SW) is limited. The equation below furtherillustrates the relationship between input current I_(IN), offsetcurrent I_(OFFSET), and switch current I_(SW).I _(SW) =I _(IN) +I _(OFFSET)  (EQ. 6)

Based on the above relationship, as offset current I_(OFFSET) isincreased input current I_(IN) will decrease. Since input current I_(IN)is directly related to the amount of power transferred during eachswitching period T_(S), when input current I_(IN) is reduced due tooffset current I_(OFFSET), less energy is transferred by energy transferelement during each switching period T_(S). In this manner, maximumenergy delivery through energy transfer element 112 is controlled by theamount of offset current I_(OFFSET) flowing through switch 104 duringeach switching period T_(S). Since, in one example, the magnitude ofoffset current I_(OFFSET) is determined in response to the magnitude ofinput voltage V_(IN), the energy delivery through energy transferelement 112 per switching period T_(S) is effectively controlled inresponse to input voltage V_(IN).

Furthermore, as offset current I_(OFFSET) is adjusted so too is thecurrent sense signal I_(SENSE). For example, based on EQ. 6 above, asoffset current I_(OFFSET) is increased, input current I_(IN) willdecrease. Since current sense signal I_(SENSE) is representative of theswitch current I_(SW), the current sense signal I_(SENSE) will indicatethat the current limit threshold has been reached for lower peak valuesof the input current I_(IN) with the noted increase in offset currentI_(OFFSET). Thus, as the current sense signal I_(SENSE) is adjusted, sotoo is a peak magnitude of the input current I_(IN) (e.g., the peakmagnitude of input current I_(IN) is reduced as more offset currentI_(OFFSET) is generated to shift the current sense signal I_(SENSE)).Accordingly, in the illustrated embodiment, the maximum output power ofpower converter 100 may be limited by generating offset currentI_(OFFSET) to adjust the current sense signal I_(SENSE) in response tochanges in the input voltage V_(IN).

In one example, power converter 100 may be designed to limit maximumpower required to load 119. In the example power converter 100, load 119may require a maximum output voltage V_(OUT) of 5 V and a maximum outputcurrent I_(OUT) of 3 A representing a maximum output power of 15 W.However, due to customer requirements, it may be necessary to limit amaximum power that can be delivered under any scenario, to less than 25W. In one example, during a low input voltage condition the powerconverter may not be capable of outputting more than the maximum powerlimit during a fault condition. In another example, if the output ofpower converter 100 is shorted during a high input voltage condition,output power of power converter 100 is limited in accordance with theteachings of the present invention. More specifically, during high inputvoltage conditions power converter 100 may be able to output more than25 W during a fault condition if output power of power converter 100 isnot limited in accordance with the teachings of the present invention.In one example, a fault condition may be that the output of the powerconverter 100 is shorted. In another example, a fault condition may bethat controller 102 fails to receive feedback signal U_(FB). Accordingto the teachings of the present invention, an offset current I_(OFFSET)is provided to limit the amount of energy transferred during a switchingperiod T_(S) or in other words limit the power delivered to the outputof power converter 100. For example, if power converter 100 is coupledto a relatively low input voltage, for example 85 V ac, current offsetcircuit 106 may provide a minimal if not a substantially zero offsetcurrent I_(OFFSET) to switch 104. Thus, the input current I_(IN) issubstantially equal to switch current I_(SW) during each switchingperiod T_(S). Continuing with the example, if power converter 100 iscoupled to a relatively high input voltage, offset current circuit 106will provide a substantial amount of offset current I_(OFFSET) to switch104. In this manner, during a fault condition where the power converter100 loses regulation at the output and switches at maximum frequency,maximum power delivered to the output is limited by offset currentI_(OFFSET). These and other examples of current limit offsetting in apower converter are disclosed in more detail below.

FIG. 2 is a functional block diagram illustrating an example powerconverter 200, in accordance with the teachings of the presentinvention. The illustrated example of power converter 200 includes acontroller 202, a power switch 204, a current offset circuit 206, apower circuit 208, a feedback circuit 214, and a current sensor 224.Controller 202, power switch 204, current offset circuit 206, powercircuit 208, feedback circuit 214, and current sense 224 representpossible implementations of controller 102, power switch 104, currentoffset circuit 106, power circuit 108, feedback circuit 114, and currentsense 224, respectively of FIG. 1A.

The illustrated example of current offset circuit 206 includes aresistor 226 and a diode 228. The illustrated example of power circuit108 includes an auxiliary winding 230, a diode 232, a bias windingcapacitor 234, a resistor 236 and a bypass capacitor 238.

As shown in FIG. 2, resistor 226 and diode 228 are coupled in seriesbetween auxiliary winding 230 and a drain terminal D of power switch204. In particular, offset current I_(OFFSET) flows through bothresistor 226 and diode 228. In one example, auxiliary winding 230 ismagnetically coupled to primary winding 220, such that auxiliary windingvoltage V_(X) is substantially representative of input voltage V_(IN) atleast during the ON state of power switch 204. In one example, thevoltage at the anode of diode 228 is substantially equal to auxiliarywinding voltage V_(X) with respect to the voltage at the anode of diode232. In one example, current offset circuit 206 supplies offset currentI_(OFFSET) only during the ON state of power switch 204.

As is further shown in FIG. 2, controller 202 and power switch 140 couldform part of an integrated circuit 205 that is manufactured as either ahybrid or a monolithic integrated circuit. Current sense 224 may alsoform part of integrated circuit 205. In power converter 200, bypasscapacitor 238 is coupled to a bypass terminal of integrated circuit 205.In operation, bias winding capacitor 234 is coupled to provide energy tobypass capacitor 238.

FIG. 3 is a functional block diagram illustrating an example powerconverter 300, in accordance with the teachings of the presentinvention. The illustrated example of power converter 300 includes acontroller 302, a power switch 304, a current offset circuit 306, apower circuit 308, a feedback circuit 314, and a current sensor 324.Controller 302, power switch 304, current offset circuit 306, powercircuit 308, feedback circuit 314, and current sense 324 representpossible implementations of controller 102, power switch 104, currentoffset circuit 106, power circuit 108, feedback circuit 114, and currentsense 224, respectively of FIG. 1. The illustrated example of currentoffset circuit 306 includes a resistor 326, a Zener diode 327 and adiode 328.

The structure of power converter 300 is similar to that of powerconverter 200 described above with reference to FIG. 2, with theexception of the addition of Zener diode 327 coupled in series withresistor 326 and diode 328. With changes in temperature of powerconverter 300, the drain-to-source resistance R_(DS) of power switch 304may correspondingly change. For example, as temperature increases, thedrain-to-source resistance R_(DS) may also increase. Therefore, as thedrain-to-source resistance R_(DS) increases the offset currentI_(OFFSET) may be undesirably reduced with increasing temperatures.Thus, Zener diode 327 may be added to current offset circuit 306 ofreduce or eliminate the effects of resistance changes of R_(DS) in powerswitch 304 due to temperature variances. More specifically, Zener diode327 may include a breakdown voltage that decreases as temperature ofswitch 304 increases, thus maintaining a substantially constant offsetcurrent I_(OFFSET). In other words Zener diode 327 is selected to have anegative temperature coefficient that negates the positive temperaturecoefficient of the R_(DS) of power switch 104. A temperature coefficientmay be defined as the amount of change in the value of a parameter. Forexample, a resistance having a positive temperature coefficient wouldsee an increase in resistance with rise in temperature and a resistancehaving a negative temperature coefficient would see decrease inresistance with rise in temperature.

Typically, Zener diodes with a breakdown voltage of less than or equalto about 6.2 volts have a negative temperature coefficient. Therefore,in one example multiple Zener diodes 328 (not shown) may be included inoffset circuit 306 coupled in series to create a greater breakdownvoltage to permit offset current I_(OFFSET) to flow to power switch 104while still incorporating this temperature compensation technique. Inone example, according to the teachings of the present invention, Zenerdiode 327 has a breakdown voltage that is less than about 6.2 volts.

FIG. 4A is a graph illustrating an effect of a current limit offsetI_(OFFSET) on switch current I_(SW), in accordance with the teachings ofthe present invention. As can be seen from FIG. 4A, switch currentI_(SW) is substantially equal to the sum of input current I_(IN) andcurrent limit offset I_(OFFSET). As can be further seen from FIG. 4A, ifcurrent limit offset I_(OFFSET) were increased, the peak value of inputcurrent I_(IN) (i.e., peak input current I_(IN) _(—) _(PEAK)) necessaryfor switch current I_(SW) to reach a threshold current limit I_(LIM)would be further reduced.

FIG. 4B is a graph illustrating a relationship between input voltageV_(IN) and current limit offset I_(OFFSET), in accordance with theteachings of the present invention. As can be seen for FIG. 4B, currentlimit offset I_(OFFSET) may be configured to adjust linearlyproportional to input voltage V_(IN). Thus, as the input voltage V_(IN)to a power converter increases the amount of input current I_(IN) neededto reach current limit threshold I_(LIM) is correspondingly reduced.Thus, the operation of a power converter (e.g., power converter 100,200, 300, etc.) may be configured to safely operate with multiple rangesof input voltages. It is appreciated that in other examples therelationship between I_(OFFSET) and V_(IN) shown in FIG. 4 b could benon-linear in nature or could be a piecewise linear characteristic.

FIG. 5 is a functional block diagram illustrating an example powerconverter 500 and an example controller 502, in accordance with theteachings of the present invention. The illustrated example of powerconverter 500 includes a controller 502, a power switch 504, a currentoffset circuit 506, a power circuit 508, an energy transfer element 512,a feedback circuit 514, an output diode 516, an output capacitor 518, acurrent sensor 524, and a power limit switch 540.

In one example, controller 502 is an integrated circuit controller withexternal terminals (e.g., including but not limited to 503, 505, 507,509, and 511.) As shown in the example of FIG. 5, integrated circuitcontroller 502 includes a drive terminal 503, a current sense terminal505, a current limit offset terminal 507, a supply terminal 509 and afeedback terminal 511.

As shown in FIG. 5, drive terminal 503 is coupled to provide a switchingsignal U_(SW) to power switch 504 in one example to regulate either orboth of an output voltage V_(OUT) or an output current I_(OUT) of powerconverter 500. Current sense terminal 505 is coupled to receive thecurrent sense signal U_(SENSE) from current sense 524. In one example,current sense signal U_(SENSE) is representative of switch currentI_(SW) flowing through power switch 504. Feedback terminal 511 is shownas coupled to receive feedback signal U_(FB) from feedback circuit 514.In one example, feedback signal U_(FB) is representative of an outputvoltage V_(OUT) of power converter 500.

Controller 502, may regulate the output of power converter 500 inresponse to feedback signal U_(FB). Power converter controller 502 mayalso implement a current limit control responsive to current sensesignal U_(SENSE) where controller 502 disables power switch 504 frombeing switched if switch current I_(SW) reaches a current limitthreshold. However, as discussed above, the current limit of controller502 may be offset by current offset circuit 506. The inclusion ofcurrent limit offset terminal 507 and power limit switch 540 allowscontroller 502 to selectively disable or enable current offset circuit506. For example, in FIG. 5, power limit switch 540 is coupled betweenpower circuit 508 and current offset circuit 506. Thus, when power limitswitch 540 is enabled current offset circuit 506 is powered, while it isunpowered when switch 540 is not conducting (i.e., open).

In one example, controller 502 enables power limit switch 540 inresponse to controller 502 regulating a maximum output power of at theoutput power converter 500. Controller 502 may also enable power limitswitch 540 to limit power delivery in response to a fault conditiondetected by controller 502. In another example, controller 502 may beconfigured to enable power limit switch 540 in response to an ambienttemperature condition of power converter 500 (e.g., temperature toohigh.)

FIG. 6 is a functional block diagram illustrating an example powerconverter 600, in accordance with the teachings of the presentinvention. The structure and operation of power converter 600 is similarto that of power converter 100 described above with reference to FIG.1A, with the exception that power circuitry 108 includes couplingcurrent offset circuit 606 to an input terminal 607. In particular,current offset circuit 606 is coupled to be powered directly from inputvoltage V_(IN) and to provide current offset signal I_(OFFSET) that isresponsive to a magnitude of input voltage V_(IN).

As discussed above, a current offset circuit may be included in a powerconverter to allow the power converter to safely limit a maximum outputpower when the power converter operates over multiple ranges of inputvoltages V_(IN). For example, a power converter, in accordance with theteachings of the present invention, may be configured, through design ofcurrent offset circuit 506, to operate with a first input voltage rangeof 85 V ac to 132 V ac and with a second input voltage range of 170 V acto 235 V ac without the need for a controller that includes a dedicatedpin for detecting the magnitude of the input voltage or a dedicated pinfor adjusting a power switch current limit threshold.

The above description of illustrated examples of the present invention,including what is described in the Abstract, are not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible without departing from the broader spirit and scope of thepresent invention. Indeed, it is appreciated that the specific voltages,currents, frequencies, power range values, times, etc., are provided forexplanation purposes and that other values may also be employed in otherembodiments and examples in accordance with the teachings of the presentinvention.

These modifications can be made to examples of the invention in light ofthe above detailed description. The terms used in the following claimsshould not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope is to be determined entirely by the following claims, which are tobe construed in accordance with established doctrines of claim1nterpretation. The present specification and figures are accordingly tobe regarded as illustrative rather than restrictive.

1. A power converter, comprising: an energy transfer element having aninput winding coupled to an input of the power converter, an outputwinding coupled to an output of the power converter, and an auxiliarywinding magnetically coupled to the input winding; a switch coupled tothe input winding to control a transfer of energy from the input to theoutput of the power converter; a controller coupled to switch the switchbetween an ON state and an OFF state to regulate the output of the powerconverter, wherein the controller is adapted to terminate the ON stateof the switch in response to a switch current flowing through the switchreaching a switch current threshold, and wherein the auxiliary windingis adapted to generate an auxiliary winding voltage that isrepresentative of an input voltage of the power converter only duringthe ON state of the switch; and a current offset circuit coupled to theauxiliary winding to generate an offset current to flow through theswitch during the ON state of the switch in response to the auxiliarywinding voltage, wherein an input current of the power converter isadjusted in response to the offset current.
 2. The power converter ofclaim 1, wherein a peak magnitude of the input current is adjusted inresponse to the offset current.
 3. The power converter of claim 1,wherein the current offset circuit is further coupled to the switch andwherein a sum of the offset current and the input current during the ONstate of the switch is substantially equal to the switch current.
 4. Thepower converter of claim 1, wherein the current offset circuit isfurther coupled to supply the offset current only during the ON state ofthe switch.
 5. The power converter of claim 1, further comprising acurrent sensor coupled to sense the switch current and to provide acurrent sense signal representative of the switch current to thecontroller.
 6. The power converter of claim 1, wherein the auxiliarywinding is included in a power circuit coupled to provide a power signalto power the controller.
 7. The power converter of claim 1, wherein thecurrent offset circuit comprises a resistor and a diode.
 8. The powerconverter of claim 7, wherein the resistor and the diode are coupledsuch that the offset current flows through both the resistor and thediode.
 9. The power converter of claim 7, wherein the current offsetcircuit further comprises a zener diode coupled such that the offsetcurrent flows through the resistor, the diode and the zener diode. 10.The power converter of claim 9, wherein the zener diode has a negativetemperature coefficient.
 11. The power converter of claim 9, wherein abreakdown voltage of the zener diode decreases as temperature increases.12. The power converter of claim 11, wherein the breakdown voltage ofthe zener diode is less than about 6.2 Volts.
 13. The power converter ofclaim 7, wherein the auxiliary winding voltage is a voltage at an anodeof the diode relative to a ground terminal of the controller.
 14. Thepower converter of claim 1, wherein the controller is adapted toimplement a current limit control technique.
 15. The power converter ofclaim 1, further comprising a feedback circuit coupled to output afeedback signal to the controller, wherein the feedback signal isresponsive to an output voltage at the output of the power converter.16. The power converter of claim 1, wherein the controller is adapted toregulate a constant voltage over a range of output currents.
 17. Thepower converter of claim 1, wherein the controller is adapted to operatein a continuous conduction mode during a first input voltage of thepower converter and to operate in a discontinuous conduction mode duringa second input voltage, wherein the second input voltage is greater thanthe first input voltage.
 18. The power converter of claim 1, wherein thecontroller is adapted to operate in a discontinuous conduction mode whenan input voltage of the power supply is substantially low.
 19. The powerconverter of claim 1, wherein the current offset circuit is adapted toadjust the offset current linearly in response to a change in the inputvoltage of the power converter.
 20. The power converter of claim 1,wherein the switch and the controller are integrated into a singlemonolithic integrated device.